1. Field of the Invention
The present disclosure relates to implantable medical devices and more particularly, implantable medical devices that provide biventricular pacing.
2. Description of the Related Art
Implantable medical devices (“IMDs”) provide various cardiac therapies including pacing, cardioversion and defibrillation. Initially, the primary purpose of a pacing therapy was to prevent symptomatic bradycardia due to disorders of impulse formation (i.e., sinus node dysfunction) or propagation (i.e., atrioventricular block).
Pacing systems evolved with the goal of providing full control of the electrical timing of the heart in order to more closely mimic the precise electromechanical relationships of the cardiac cycle. For example, in a conventional dual chamber pacing system (right atrial (RA) and right ventricular (RV) pacing), a dual chamber pacing mode such as DDD/R could provide atrial timing to control heart rate in response to sensed physiological demand (i.e., increasing or decreasing heart rate as needed) and provide properly timed atrial-synchronous RV pacing to assure atrioventricular (AV) synchrony. In this manner, the various timing parameters were well defined and controlled. That is, the pacemaker AV delay (AVD: time from sensed or paced RA event to a RV paced event unless inhibited by a spontaneous ventricular event) was set to a physiologically desirable value that maximized ventricular pump function during systole and the corresponding ventriculo-atrial (VA) interval was sufficiently long to permit ventricular relaxation and filling during diastole. In general, the implantation of a dual chamber device is relatively straightforward and the implementation and operation of such a conventional device in a dual chamber mode (e.g., DDD, DDD/R, etc.) became a gold standard for therapy.
Awareness of the detrimental effects of electrical timing disturbances on cardiac pump function encouraged the use of multisite pacing stimulation as electrical therapy for other types of cardiac disease. As an example, many systolic heart failure patients with intact AV conduction and proper intrinsic rate control also have ventricular conduction disturbances which cause regional mechanical delay (contraction asynchrony) that worsens pump function. Cardiac resynchronization therapy (CRT) was developed to provide pacing stimuli to both the RV and left ventricle (LV) (and typically the RA) to restore and maintain AV synchrony similar to conventional dual chamber pacemakers, and additionally, to restore and maintain ventricular contraction synchrony using biventricular (RV+LV) stimulation techniques. CRT is an important and established therapy for systolic heart failure accompanied by ventricular conduction delay resulting in LV contraction asynchrony.
Any technique of biventricular pacing requires delivery of a specially designed pacing lead to the epicardial or endocardial surface of the LV. This can be achieved by several different approaches, including transvenous epicardial, transvenous endocardial and direct epicardial via limited left lateral thoracotomy. Generally, LV pacing leads are delivered transvenously to the RA, navigated through the coronary sinus into a coronary vein tributary adjacent an exterior portion (epicardial surface) of the LV. Currently, application of biventricular pacing is confined to patients with systolic heart failure and ventricular conduction delay that occurs spontaneously or is imposed by obligatory RV pacing.
Conventional dual chamber pacing prioritizes AV timing by synchronizing a RV paced beat to every RA event (paced or sensed), even when AV conduction is intact. Over time, there has been recognition in the field that RV apical (RVA) pacing (i.e., only providing ventricular pacing in the right ventricular apex), despite maintenance of AV synchrony, is associated with increased risks of atrial fibrillation, heart failure and death. These adverse effects are attributed to disruptions to AV timing and an asynchronous ventricular contraction sequence obligated by RVA-only pacing. In many patients obligatory RVA-pacing is a consequence of inviolable pacemaker timing rules. Consequently, new pacemaker modes were developed to prioritize intrinsic AV conduction and spare RVA pacing, permitting significantly prolonged AV conduction times and occasional missed ventricular beats.
Accordingly, a pacing protocol to promote intrinsic conduction has been developed having a protocol that is commercialized in various embodiments as MVP™ (Managed Ventricular Pacing™). In summary, MVP™ operates to provide a pacing protocol that provides multiple-beat AV synchronization and minimizes or reduces RV pacing by (1) eliminating the pacemaker AV interval (AVI) and restrictions on the PR interval and (2) occasionally tolerating a complete cardiac cycle devoid of ventricular activity while prioritizing intrinsic AV conduction and ventricular contraction synchrony. In other words, the protocol tolerates a complete cardiac cycle devoid of ventricular activity (i.e., no sensed spontaneous ventricular event, no ventricular pacing) while providing properly timed atrial synchronous ventricular pacing in the cycle immediately subsequent to the cardiac cycle devoid of spontaneous ventricular activity. If there is a loss of intrinsic conduction for a prolonged period of time, the protocol will cause the device to operate in a dual chamber mode (e.g., DDD/R) to maintain AV synchrony and prevent ventricular asystole, and to periodically perform conduction checks for recovery of intrinsic AV conduction. Minimization of unnecessary RVA pacing using such techniques has been shown to reduce the risks of atrial fibrillation and heart failure compared to obligatory RVA pacing at standard AV delay settings associated with conventional dual chamber pacemakers.
While these kinds of pacing protocols provide ever safer and efficacious therapies for various patient populations, there remain numerous other patient populations that lack an effective pacing therapy to address their cardiac condition.